1. Field of the Invention
The present invention relates to a communication apparatus and a communication system.
2. Description of the Related Art
Data communication has recently become more and more important with the spreading Internet, etc. as its background. ATM networks in accordance with ITU-T 1.150 have been introduced in this field to integrate telephony and data communication. High-speed optical communication network technologies such as SONET defined by ANSI T1.105, SDH defined by ITU-T G.707, and OTN defined by ITU-T G.872 have been introduced in this field. SONET and SDH have basically the same function, and OTN, an optical network, is based on the WDM (Wavelength Division Multiplexing) technology to raise the transmission capacity in a fiber.
These networks are interconnected by prescribing upper and lower layers based on the layer hierarchy of the OSI (Open System Interconnection) reference model defined by ISO. According to the provisions of these individual networks, ATM is ranked at the top; OTN, at the bottom; SONET and SDH, at the middle. Each network is also of hierarchic structure, consisting of subnetworks. For example, an ATM network consists of a virtual path and a virtual channel, and SONET/SDH and OTN are of hierarchic structures, consisting of subnetworks, as well.
Table 1 shows the relations between such upper and lower layers as relate to switching in the above subnetworks, segments defined by repeaters excluded.
FIG. 1 shows the relation between defined segments and multiplexing in a lower and an upper layer. Numerals 100-102 and 110-112 indicate apparatuses in the upper layer; 300 and 310, apparatuses in the lower layer; 500-502, communication lines in the upper layer; 600, a communication line in the lower layer.
In FIG. 1, a plurality of upper-layer apparatuses is connected to a lower-layer apparatus. The upper-layer apparatuses 100-102 are connected to the lower-layer apparatus 300 through the upper-layer communication lines 500-502. The upper-layer apparatuses 110-112 are connected to the lower-layer apparatus 310 through the upper-layer communication lines 500-502. The lower-layer apparatuses bundle the upper-layer communication lines by TDM (Time Division Multiplexing) of SONET of ANSI T1.105 or SDH of ITU-T G.707 or by WDM of OTN of ITU-T G.872 so as to raise the data transmission capacity per physical medium or unit time and transmit data through the lower-layer communication line.
The upper-layer apparatuses 100 and 110 process the overhead portions of SONET or SDH signals when they transmit and receive the signals through the upper-layer communication line 500, the overhead portions containing the administration control signals of the upper-layer communication line. The upper-layer apparatuses 101 and 111, and 102 and 112 carry out the same processing for the upper-layer communication lines 501 and 502, respectively. The segment for processing the administration control signals of the upper-layer communication line is called upper-layer communication-line segment (hereinafter xe2x80x9cupper-layer segmentxe2x80x9d). The upper-layer communication lines 500-502 may be three physically separate lines or such a physically single line comprising three separate logical lines as the bus of SONET or SDH. In the same way, the lower-layer communication line 600 is terminated by the lower-layer apparatuses 300 and 310, and the segment between the apparatuses is called the lower-layer communication-line segment (hereinafter xe2x80x9clower-layer segmentxe2x80x9d).
In this way, signals of the upper layer are multiplexed to become signals of the lower layers.
Accordingly, the signals of the upper layer may become equal to but never become larger than the signals of the lower layer in terms of the signal band. The defined segments of the upper layer may be equal to but never shorter than the defined segment of the lower layer.
FIG. 2 shows a case wherein each of the upper and lower layers has protection-switching processing parts. Upper-layer communication lines 500 to 505 are terminated by upper-layer apparatuses 100-104 and signal processing parts 120-124 and 125-129 in the apparatuses 100-104. Multiplexing/demultiplexing parts 440 and 450 in lower-layer apparatuses 300 and 310 multiplex and demultiplex the upper-layer communication lines 500-503, which, going through the lower-layer segment of the lower-layer communication line 600 or 601, connect the upper-layer apparatuses 100-103 with those 110-113. In the same way, multiplexing/demultiplexing units 441 and 451 in lower-layer apparatuses 301 and 311 multiplex and demultiplex the upper-layer communication lines 504-505, which, going through the lower-layer segment of the lower-layer communication line 602 or 603, connect the upper-layer apparatuses 103 and 104 with those 113 and 114.
For the sake of simplification of description, a protection switching system of the 1+1 type will be taken as an example and its description will follow. The system comprises a set of a working communication line and a protection line. While the system is operating normally, the same data are transmitted through both the working and protection lines and the receiving side chooses a line of which the transmission quality is better than that of the other. In FIG. 2, a lower-layer communication-line bridge/selector part 420 or 430 on the transmitting side transmits the same data through both the working and protection lower-layer communication lines 600 and 601. In the same way, a lower-layer communication-line bridge/selector part 421 or 431 on the transmitting side transmits the same data through both the working and protection lines 602 and 603. While all the communication lines and all the apparatuses are normal, the lower-layer communication-line bridge/selector part 430 or 420 on the receiving side chooses the working line 600. When the working line 600 goes out of service due to some failure, the lower-layer communication-line bridge/selector part 430 or 420 on the receiving side chooses the protection line 601 to restore the communication line in the lower layer. In the same way, while all the communication lines and all the apparatuses are normal, the lower-layer communication-line bridge/selector part 431 or 421 on the receiving side chooses the working line 602. When the working line 602 goes out of service due to some failure, the lower-layer communication-line bridge/selector part 431 or 421 on the receiving side chooses the protection line 603 to restore the communication line in the lower layer.
As in the case of the lower-layer communication lines, an upper-layer communication-line bridge/selector part 220 or 230 on the transmission side transmits the same data through both the working and protection upper-layer communication lines 503 and 504. While all the lines and all the apparatuses are normal, the upper-layer communication-line bridge/selector part 230 or 220 on the receiving side chooses the working line 503. When the working line 503 goes out of service, the upper-layer communication-line bridge/selector part 230 or 220 on the receiving side chooses the protection line 504 to restore the communication line.
Describe below is the switching processing in the upper and lower layers upon the occurrence of failure in the lower-layer communication line 600 as shown in FIG. 3. The switching method was devised to describe the embodiments of the present invention. FIG. 3 shows a portion including the protection-switching processing parts of FIG. 2. In this method, when failure has occurred in the lower layer, an alarm indication signal (AIS) is sent to the upper-layer apparatuses. FIG. 4 shows the concept of AIS. As shown in FIG. 4, upper-layer communication lines 500-502 are terminated by upper-layer apparatuses 100-102 and 110-112. Lower-layer apparatuses 300 and 310 connect the upper and lower layers, and the lower-layer apparatuses 300 and 310 are connected to each other by a lower-layer communication line 600. Now it is assumed that failure has occurred in the lower-layer communication line 600. In FIG. 4, signals transmitted in both directions of the lower-layer communication line 600 are shown. Now a case wherein the failure has effects in both directions is considered. At first, each of the lower-layer apparatuses 300 and 310 detects the failure in the lower-layer communication line 600 and sends out an AIS through all the upper-layer communication lines 500-502. Accordingly, all the upper-layer apparatuses 100-102 and 110-112 connected to the lower-layer apparatuses recognize that failure occurred in the lower-layer communication line 600 and the data transmitted through the upper-layer communication lines 500-502 are invalid. The AIS is prescribed in each layer of each network shown in Table 1; i.e., the virtual path and the virtual channel of the ATM network, the path and the line of the SONET, the path and the M section of the SDH network, the optical channel and the optical multiple section of the OTN network, and so on. The provisions of each network prescribe the AIS from the lower layer to the upper layer in each network in accordance with Table 1. Internetwork AIS is also prescribed. For instance, ITU-T 1.610 prescribes the AIS for the case wherein the lower layer is a SONET and the upper layer is an ATM network.
FIG. 5 is a conceptual illustration of the AIS from a lower layer to an upper layer. The line connecting the upper layer and the lower layer is the upper-layer communication line, and the line connecting the two lower-layer apparatuses of the lower layer is the lower-layer communication line. AIS is made from the lower layer to the upper layer but not from the upper layer to the lower layer.
With this AIS system, the upper layer can recognize the failure which has occurred in the lower layer. In this method, the protection-switching processing part of each of the upper and lower layers have independent protocol and determines independently whether to switch the communication line or not based on failure information detected or an AIS received, as the case may be. When failure has occurred in the-lower-layer communication line 600 of FIG. 3, the lower-layer apparatuses 300 and 310 detect the failure and begin the switching processing to switch the communication line from the working line 600 to the protection line 601 in the lower layer. On the other hand, the upper-layer apparatuses 100 and 110 begin the switching processing upon their receipt of an AIS to switch the communication line from the working line 503 to the protection line 504 in the upper layer. In this way, line switching takes place in both the lower and upper layers even when line switching is required in only one layer for the restoration of a failed line. To prevent such unnecessary line switching, either the lower-layer switching function or the upper-layer switching function may be disenabled permanently. For example, ANSI T1.105 prescribes NUT (Non-preemptible Unprotected Traffic) to prohibit line switching path by path of SONET apparatuses, assuming that a SONET apparatus is connected to a network with a switching function such as an ATM network, the former constituting the lower layer and the latter constituting the upper layer.
Another shortcoming anticipated with the system wherein switching is made in both the upper and lower layers is that while the lower-layer apparatuses are switching the communication line, the upper-layer apparatuses may not correctly send and receive the switching protocol between them. One method of coping with this problem would be to stop the switching processing in either of the two layers for a prescribed time period. For instance, ITU-T I.630 prescribes the hold-off time for the system consisting of an ATM network and a SONET or SDH network. While the SONET or SDH network constituting the lower layer is switching the communication line, the switching processing of the ATM network constituting the upper layer is stopped for the hold-off time, of which the range and increment are 0-10 seconds and 500 msec.
As described above, a loss occurs in the switching time because the switching in one of the two layers has to be stopped permanently or for a prescribed time period in case of a system consisting of a lower layer and an upper layer, each having a line-switching function. In the case of the NUT method of ANSI T1.105, the protection-switching processing parts of a SONET become useless because the switching in the network is stopped. On the other hand, according to the method of ITU-T I.630, there occurs a waiting time of at least 500 msec in case that failure which cannot be dealt with in a SONET is to be dealt with in an ATM network. FIG. 7 is the time chart of this method. When failure occurs in the lower layer, the lower-layer apparatuses detect it and send out an AIS through all the upper-layer communication lines. Accordingly, the upper-layer apparatuses connected with the lower-layer apparatuses recognize that failure occurred in the lower-layer communication line and the data transmitted through the upper-layer communication lines are invalid. However, because a hold-off time is set in the upper-layer apparatuses to prevent unnecessary switching as described above, the upper-layer apparatuses do not begin the switching processing during the hold-off time. In FIG. 7, it is assumed that the upper layer is an-ATM network, the lower layer is a SONET or SDH network, and the hold-off time is 500 msec.
The lower-layer apparatuses send out the AIS to the upper layer and, at the same time, begin to switch the communication line. Because the protection switching system is of a 1+1 type, the receiving apparatuses that detected the failure in the communication line have their bridge/selector parts switch the communication line from the working line to the protection line. In the case of SONET, it is prescribed that the time necessary for the line switching should be 50 msec or less. If the protection line also has trouble or the bridge/selector apparatuses do not function correctly due to trouble of an apparatus, the line switching in the lower layer cannot be made. Namely, the lower-layer receiving apparatuses keep detecting the failure in the lower-layer communication line and sending out the AIS through the upper-layer communication lines. But, the upper-layer apparatuses continue standing by until the hold-off time passes. Then, the upper-layer apparatuses carry out the switching processing. Namely, the upper-layer receiving apparatuses have their bridge/selector parts switch the communication line from the working line to the protection line in the upper layer. In the case of ATM, the target value of the switching time of 50 msec is prescribed in ITU-T Draft New Recommendation I.630. After the line switching has been made successfully, the upper-layer apparatuses can receive data through the protection line without detecting failure or receiving an AIS. As shown in FIG. 7, SONET switches the communication line within 50 msec. However, the difference of 450 msec between the hold-off time of 500 msec and the SONET switching time of 50 msec are wasted because action is taken in neither the upper nor the lower layer.
The object of the embodiments of the present invention is to achieve interlayer-coordinated communication-line switching in a communication system or network comprising two layers, each having a switching function. In the interlayer-coordinated communication-line switching, the features of the two switching functions are utilized according to the line conditions so that the interlayer-coordinated communication-line switching can be made in a shorter time and is more efficient and reliable than the switching in accordance with the prior art.
An embodiment of the present invention is as follows, and part of the configuration of the embodiment is shown in FIG. 21. A communication system wherein (1) provided are a first and a second upper-layer apparatus and a first, a second, a third, and a fourth lower-layer apparatus, each apparatus having line-switching means, (2) (i) the first and second upper-layer apparatuses are connected to each other through a first and a second upper-layer communication line between which communication can be switched, (ii) the first and second lower-layer apparatuses are connected to each other through a first and a second lower-layer communication line between which communication can be switched, each lower-layer communication line being multiplexed to accommodate one or more upper-layer communication lines, (iii) the third and fourth lower-layer apparatuses are connected to each other through a third and a fourth lower-layer communication line between which communication can be switched, each lower-layer communication line being multiplexed to accommodate one or more upper-layer communication lines, (iv) the first upper-layer communication line connects the first upper-layer apparatus with the first lower-layer apparatus and the second upper-layer apparatus with the second lower-layer apparatus, and (v) the second upper-layer communication line connects the first upper-layer apparatus with the third lower-layer apparatus and the second upper-layer apparatus with the fourth lower-layer apparatus, (3) a switching-inhibit notification is transmitted to the upper-layer apparatuses when the lower-layer apparatuses have detected line failure, (4) one or more faulty lines of the lower or the upper layer are identified and the site of occurrence of failure is located by using failure information on the lower- and upper-layer communication lines, (5) the switching-inhibit signal to the upper-layer apparatuses is cancelled when no lower-layer communication line has been found faulty but any of the upper-layer communication lines has been found faulty, (6) which line-switching means of the lower- and upper-layer apparatuses should make switching is determined, based on failure information on the lower- and upper-layer communication lines, in order to secure a largest number of normal upper-layer communication lines, or in order to restore high-priority lines rather than low-priority lines, or in order to secure a largest number of signal channels, in case that any of the lower-layer communication lines has been found faulty, and (7) provided is a first means for causing line switching in the lower layer first and then canceling the switching-inhibit signal to the upper-layer apparatuses, in case that switching is to take place in both the lower and upper layers in accordance with said determination.